Ecologists at the University of California, San Diego, offer a new
explanation for an apparently abrupt switch in the kinds in of mammals
found along the Malay Peninsula in southeast Asia – from mainland
species to island species – in the absence of any geographical barrier.

An ancient seaway between the Gulf of Thailand and the Andaman Sea
was once thought have split the peninsula in two, allowing separated
populations of animals to diverge. But a recent revision of the history
of sea levels reveals that the ocean hasn’t cut through the peninsula
in the past 40 million years. Current species of mammals are much
younger than that.

Instead, David Woodruff, professor of biology at UC San Diego and
former graduate student Leslie Turner, now at the Max Planck Institute
for Evolutionary Biology in Ploen, Germany, say that more than 58 rapid
sea level rises in the last 5 million years could account for the shift
by crowding out species, particularly at the narrowest part of the
peninsula called the Isthmus of Kra. The Journal of Biogeography posted their findings online February 25.

For most of the past few million years, the shallow ocean shelf
surrounding the peninsula and islands of Malaysia and Indonesia has
been exposed, creating a land area about the size of Europe. That
habitat shrank dramatically each time sea levels rose.

“The ocean is coming from both sides repeatedly to squeeze things,”
Woodruff said. “If you have the ocean edge coming back and forth more
than 50 times around this little narrow area, you’ll compress the fauna
and cause species to go extinct locally.”

By consulting published reports, Woodruff and Turner mapped the
ranges of 325 species of mammals found in the region. “We studied
mammals from China down to Singapore,” Woodruff said. But they found no
evidence for a sharp transition between types of mammals at Kra or
anywhere else along the peninsula.

Instead, they saw a gap. “We found an area about 600 kilometers long
where there are 30 percent fewer species than there should be,”
Woodruff said. Although 128 mainland Asian species stopped before the
southern tip of the peninsula and 121 island species were found only in
the south, 35 widely distributed species were found above and below the
isthmus, but were missing from that narrowest part.